How to Repair, Restore, or Reinstall Grub 2 with a Ubuntu Live CD or USB

source:http://howtoubuntu.org/how-to-repair-restore-reinstall-grub-2-with-a-ubuntu-live-cd#.Ui5w2pEgi90

How to Repair, Restore, or Reinstall Grub 2 with a Ubuntu Live CD or USB

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Grub 2 typically gets overridden when you install Windows or another Operating System. To make Ubuntu control the boot process, you need Reinstall (Repair/Restore) Grub using a Ubuntu Live CD.

Warning:

Using the sudo command, especially from a Live CD can do serious damage to your system. Read all instructions and confirm you understand before executing any commands. When pasting into the Terminal, use Ctrl+Shift+V, NOT Ctrl+V.

Terminal Commands:

Mount the partition your Ubuntu Installation is on. If you are not sure which it is, launch GParted (included in the Live CD) and find out. It is usually a EXT4 Partition. Replace the XY with the drive letter, and partition number, for example: sudo mount /dev/sda5 /mnt.

sudo mount /dev/sdXY /mnt

Now bind the directories that grub needs access to to detect other operating systems, like so:

sudo mount --bind /dev /mnt/dev && sudo mount --bind /dev/pts /mnt/dev/pts && sudo mount --bind /proc /mnt/proc && sudo mount --bind /sys /mnt/sys

Now we jump into that using chroot.

Now install, check, and update grub.

This time you only need to add the drive letter (usually a) to replace X, for example: grub-install /dev/sda, grub-install –recheck /dev/sda.

grub-install /dev/sdXgrub-install --recheck /dev/sdX

Now grub is back, all that is left is to exit the chrooted system and unmount everything.

exit && sudo umount /mnt/dev && sudo umount /mnt/dev/pts && sudo umount /mnt/proc && sudo umount /mnt/sys && sudo umount /mnt

Shut down and turn your computer back on, and you will be met with the default Grub2 screen.

You may want to update grub or re-install burg however you like it.

Congratulations, you have just Repaired/Restored/Reinstalled Grub 2 with a Ubuntu Live CD!